1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to gate drive circuits for high frequency, switched mode electronic power converters. This application is a continuation-in-part of pending application Ser. No. 10/878,448 filed 2004 Jun. 28. Some of the subject matter of this application was first revealed in Disclosure Document Number 527396.
2. Description of Related Art
One simple gate drive circuit that has seen use in some high volume commercial applications is shown in FIG. 1. This circuit was invented by the applicant, but no patent protection was sought by the assignee. Here an inverted PWM control signal is capacitively coupled to a high side main switch in a buck converter. The gate circuit of the high side switch contains an inverting driver that inverts the gate drive signal a second time resulting in a non-inverted gate drive signal, as desired. The key to the understanding of this simple circuit is the source voltage of the main high side switch. During a turn on transition of the main switch the voltage at the input to the driver U2 is falling, causing the voltage at the output of U2 and the gate of the main switch to rise. As the gate voltage of the main switch rises at the output of U2 the main switch turns on causing the source of the main switch to rise in voltage towards VLINE. As the source voltage rises the capacitor CDRIVE is charged through the diode D2, forcing the input of U2 to stay low and the output of U2 to stay high through the transition, as desired. One can see that the double inversion is absolutely necessary for the successful operation of this gate drive circuit. During a turn off transition of the main switch the input of U2 is initially driven high by CDRIVE and is forced to stay high during the transition as the source voltage falls and the diode D1 discharges the capacitor CDRIVE. In each case the movement of the source reinforces the initiating transition at the input of U2, which can only occur if U2 is an inverter, as indicated. The high side gate drive circuit is powered by the bootstrap diode DBOOT and the capacitor CBOOT, which is charged during the off time of the main switch when the synchronous rectifier conducts. This circuit, as shown, requires that the source of the main switch be at zero volts in order to accomplish charging of the capacitor CBOOT to the proper voltage. What is needed is a more general circuit that can work in applications in which the source voltage of the high side switch is different than zero volts during the off time of the high side switch.